A wide variety of fibrous materials have been employed in the production of tobacco smoke filter elements, particularly for filtered cigarettes and the like. The choice of such materials has been limited because of the need to balance various commercial requirements. A very important property of a tobacco smoke filter is obviously its filtration efficiency, i.e., its ability to remove selected constituents from the tobacco smoke. While there is no commercially acceptable retention level, the typical range is 35-60% total particulate matter. The range of filtration efficiency has often had to be comprised in order to satisfy other commercially important factors, such as resistance to draw, hardness, impact on taste and manufacturing ease and expense. For example, sometimes retention levels of 70% and higher are required; in such instances, the firmness of the filter often becomes the limiting factor. As fibers get smaller to provide higher retention, the filter elements become softer.
Cellulose acetate has long been considered the material of choice in the production of tobacco smoke filters, primarily because of its ability to provide commercially acceptable filtration efficiency, on the order of about 50%, low resistance to draw, and acceptable filter hardness without significantly detracting from the tobacco taste desired by the majority of smokers. Yet, tobacco smoke filter elements incorporating fibers comprising homopolymers of cellulose acetate have numerous limitations and disadvantages.
U.S. Pat. No. 5,509,430 issued Apr. 23, 1996 (the '430 patent), the subject matter of which is incorporated herein in its entirety by reference, is directed to the production of tobacco smoke filters comprising sheath-core bicomponent fibers with the core being a low-cost, high strength, thermoplastic material, preferably polypropylene, completely covered with a sheath formed of plasticized cellulose acetate, ethylene vinyl acetate copolymer, polyvinyl alcohol or ethylene-vinyl alcohol copolymer. Each of these sheath-forming materials provides commercially acceptable taste in tobacco smoke products. Yet, the core-forming thermolastic polymer affords the smoke-permeable matrix with significant strength so that the thickness of the more expensive sheath-forming material is limited and the cost of the product is dramatically reduced. Filter elements formed from each of the specific bicomponent fiber embodiments referred therein have unique and advantageous properties, particularly when incorporated into tobacco smoke filter products such as filtered cigarettes.
Among the various sheath-forming materials discussed in the '430 patent, an ethylene-vinyl acetate copolymer has been found to be especially useful in the production of filtered cigarettes and the like because of the highly desirable taste properties of ethylene-vinyl acetate when contacted by tobacco smoke. However, problems have been encountered in attempting to commercially process bicomponent fibers having a sheath formed entirely of ethylene-vinyl acetate copolymer. Normally, a multiplicity of fibers are subjected to a treatment with steam and then contacted with cooling air to bond the fibers at their points of contact to form a continuous rod defining a tortuous interstitial path for passage of smoke when the rod is subdivided into tobacco smoke filter plugs to be incorporated into filtered cigarettes or the like. The ethylene-vinyl acetate copolymer sheath material of such bicomponent fibers tends to stick in conventional commercial rod-forming dies. In order to deal with this problem, it was necessary to develop modified equipment utilizing an application of indirect steam which minimized the undesirable build-up of polymer in the die. Unfortunately, with such equipment, lower machine speeds were required and unsatisfactory bonding was still experienced.
In addition to the manufacturing problems encountered with processing bicomponent fibers having an ethylene-vinyl acetate copolymer sheath and a thermoplastic polymeric core such as polypropylene, poor adhesion between the sheath- and core-forming materials resulted in polymer separation at the interface. A wide range of ethylene-vinyl acetate polymers and copolymers and related materials were tested, but in each instance materials that provided satisfactory sheath-core bonding created a sticking problem in the die.
Tobacco smoke filter elements formed from bicomponent fibers with a sheath of ethylene-vinyl acetate copolymer were also found to be less firm or hard than filters formed from standard cellulose acetate fibers. While there is no commercially acceptable hardness level, the 180 minimum hardness stated in U.S. Pat. No. 3,377,220, the subject matter of which is incorporated herein by reference, is desirable, although commercial cellulose acetate filter elements having a hardness in the 160 range are in the market. Softer filter elements provide a different feel to the smoker. In extreme instances, a smoker's lips can tend to collapse the filter plug, reducing the permeability of the matrix and increasing the resistance to draw smoke through the filter element. Reduced hardness also causes problems in the processing of such elements by the high speed filtered cigarette manufacturing equipment commercially in use.